The compound class of the π-conjugated polymers has been the subject-matter of numerous publications in the last few decades. They are also referred to as conductive polymers or synthetic metals.
Conductive polymers are gaining increasing economic importance, since polymers have advantages over metals with regard to the processability, the weight and the precise adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylenevinylenes).
A particularly important and industrially utilized polythiophene is poly-3,4-(ethylene-1,2-dioxy)thiophene, often also referred to as poly(3,4-ethylenedioxythiophene), which has very high conductivities in its oxidized form and is described, for example, in EP-A 339 340. A review of numerous poly(alkylenedioxythiophene) derivatives, especially poly-(3,4-ethylenedioxythiophene) derivatives, their monomer building blocks, syntheses and applications is given by L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12 (2000) 481-494.
The European Patent EP-A 340 512 describes the production of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymers, prepared by oxidative polymerization, as a solid electrolyte in electrolyte capacitors. Poly(3,4-ethylenedioxythiophene) as a replacement for manganese dioxide or for charge transfer complexes in solid electrolyte capacitors reduces the equivalent series resistance of the capacitor owing to the higher electrical conductivity and improves the frequency performance.
JP-A 2000-021687 states that the high-frequency performance of electrolyte capacitors can be improved by the use of poly(3,4-ethylenedithiathiophene) as a solid electrolyte. However, 3,4-ethylenedithiathiophene has the disadvantage of difficult preparation (Lit.: C. Wang, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis, Chem. Mater. 1995, 7, 58-68). According to this literature method starting from 3,4-dibromothiophene, the synthesis proceeds only with an overall yield of 19%, and process disadvantages such as reaction at −78° C., use of very moisture-sensitive reagents such as n-butyllithium and metallic potassium and also of highly explosive and toxic carbon disulphide have to be accepted. Although 3,4-ethylenedithiathiophene can also be prepared from 3,4-dialkoxythiophenes and 1,2-dimercaptoethane by the principle of transetherification, this also results in extremely highly odorous sulphurous products which very markedly restrict the practicability of this synthetic route and the use of the product. In addition, the investigations of the above literature show that it was possible using 3,4-ethylenedithiathiophene to prepare polythiophenes having only moderate conductivity of 0.1 S/cm (as the tetrachloroferrate) or 0.4 S/cm (as the polymer prepared electrochemically). However, higher conductivities are desirable for a further improvement in the high-frequency performance. In comparison, the following values are obtained for polythiophene composed of ethylenedioxythiophene: 5-31 S/cm (as the tetrachloroferrate), see Lit. F. Jonas, G. Heywang, Electrochimica Acta 39 (8/9), p. 1345-1347 (1994) and 200 S/cm (prepared electrochemically), same literature. The leakage current of electrolyte capacitors having polymeric solid electrolytes is about 10 times higher than in electrolyte capacitors having manganese dioxide as the solid electrolyte (I. Horacek et al., Proceedings of the 15th European Passive Components Symposium CARTS-Europe 2001, Copenhagen, Denmark, p. 24-29). The high current losses lead, for example in mobile electronics applications, to earlier flattening of the battery. It is desirable to reduce leakage current of electrolyte capacitors using polymeric solid electrolytes.
There is therefore a need for suitable electrically conductive polymers which are suitable as solid electrolytes in electrolyte capacitors, with a view to improving the leakage current in comparison to known polymers, for example poly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedithiathiophene).
It is an object of the invention to provide electrolyte capacitors comprising such electrically conductive polymers as solid electrolytes.
In Org. Lett. 4 (4), 2002, p. 607-609, Roncali et al. describe the preparation of 3,4-ethyleneoxythiathiophene (thieno[3,4-b]-1,4-oxathiane, EOTT) and its electrochemical polymerization to give poly(3,4-ethyleneoxythiathiophene) (PEOTT). Comparison with the corresponding dioxy and dithia analogues shows that the replacement of alkoxy groups by alkylsulphanyl groups on the thiophene ring significantly reduces its polymerization potential. There is no description of conductivity data or investigations with regard to the use of the polymer in capacitors.